1. Field of the Invention
The present invention relates generally to battery separator systems and particularly to alkaline electrolyte absorbers for use, for example, in cells of high energy density secondary alkaline batteries.
2. Description of the Prior Art
An illustrative environment where application of the principles of the present invention is particularly advantageous is in high energy density batteries such as, for example, the batteries as described in a pending patent application Ser. No. 57,329 to the present co-inventor George Schmidt "Alkaline Battery, Separator Therefore" filed July 13, 1979, now U.S. Pat. No. 4,224,394, dated Sept. 23, 1980.
The cited patent describes a battery electrolyte absorber used in conjunction with a barrier type separator in a separation system designed primarily for silver-zinc and nickel-zinc alkaline electrolyte cells. The described electrolyte absorber, which is shaped to form a housing for receiving an electrode is comprised of a fuel-cell grade asbestos sheet that is about 7 mils thick which is treated in a beater with 5% by weight of butyl latex rubber. The absorber material is capable of absorbing electrolyte, holding electrolyte in contact with a housed electrode and acting as a low resistance passageway for electrolyte ion transfer. However, as is the usual situation, the absorber material by itself does not provide a sufficient barrier to electrode ion transfer. Hence, in battery cells which utilize zinc as the negative electrode material, wherein reduction of the electrode material occurs at the negative electrode during charging and wherein dendrites of sharp tooth-like projection have a tendency to build up on the negative electrode, a barrier type separator, similar to the one as described in the cited patent application is also employed to deter migration of electrode ions and resultant growth to dendrite crystals. The tendency to form dendrites is reduced when the soluble zincate ions are retained in close proximity to the negative electrode.
However, recognizing possible environmental risks in using asbestos as a substrate the present inventor saw the need for developing a low cost, high efficient polymeric material to replace the asbestos substrate.
In one system for meeting this need, the present inventors, George Schmidt and Robert Weber, developed a barrier type separator which included a wettable melt brown thermoplastic polymer web to replace the asbestos as the separator substrate. Pending patent application Ser. No. 176,991 to Schmidt and Weber "Flexible Separators for Alkaline Batteries" filed Aug. 11, 1980, which is a continuation of Ser. No. 001,924, filed Jan. 8, 1979, now abandoned, describes such a separator.
U.S. Pat. No. 3,985,580 dated Oct. 12, 1976 to Coqliano describes a method for preparing wettable polyolefin battery separators that retain wettability over long periods of use in batteries utilizing dilute sulfuric acid as the electrolyte. The separator described in this patent is preferably a self-supporting non-woven mat of polypropylene fiber having a thickness of 10-20 mils, with a soluble surfactant and some amount of colloidal silica incorporated in the non-woven mat. However, the silica, which is a colloidal filler, is soluble in alkaline solution and the described separator will not work satisfactorily in alkaline batteries.
U.S. Pat. No. 4,072,802 dated Feb. 7, 1978 to Murata et al discloses a separator for lead-acid batteries wherein a polypropylene non-woven web is made wettable after formation with a composition consisting of polyvinyl chloride, silicon oxide and tetrahydrofuran. This material allegedly provides a suitable acid electrolyte wettability of the polyolefin.
To provide an alkali-wettable melt blown thermoplastic polymer web suitable for use as an electrolyte absorber in a battery separator system, it is desirable to minimize loss of wetting agents and wetting agent carriers which may leach out or be removed by agitation. Also, it is desirable, with a substantially permanently wettable material to employ a wetting composition which allows for rapid wetting of the material providing a very low resistance to electrolyte ion transfer.
To obtain the above-mentioned desiderata, a search for various other means to enhance the wetting characteristics of polymeric material in alkaline batteries was initiated. This search resulted in the improved product of the present invention.